Week Eleven: Meiosis and Inheritance

Question 1

Which statement describes a phenotype

Answer 1

the physical trait produced by an organism’s genetic makeup

Question 2

Cell cycle of a somatic vs. reproductive cell

Answer 2

Somatic cells: made by mitosis
Reproductive cells: made by meiosis

Question 3

Mitosis produces genetically identical cells for

Answer 3

• Growth
• Replacement
• Asexual reproduction

Question 4

Diploid vs. Haploid

Answer 4

*23 pairs–––each matched pair is said to be homologous
*23 homologous pairs or 46 total chromosomes
*Zygote: diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum
*Haploid = 1n
- contains 1 copy of every chromosome
*Diploid = 2n
- contains 2 copies of every chromosome (i.e. homologous chromosomes)
*Diploid cells with homologous chromosome pairs divide to produce haploid cells with no pairs of chromosomes.

Question 5

1. Prophase I

Answer 5

*The nuclear envelope dissolves
*Centrosomes move to the poles and from the spindle apparatus
*Chromosomes condense and form homologous pairs
*Homologous chromosomes pair (synapsis) and form a tetrad or bivalent

Question 6

2. Metaphase I

Answer 6

*Homologous chromosome pairs line up in the center of the cell
*Homologous pairs can line up in any order (independent assortment)
*This is another diversity-creating mechanism in meiosis

Question 7

3. Independent Assortment

Answer 7

*Independent assortment is when homologous chromosome pairs align at the metaphase plate
*When the homologous pairs align at the plate, the maternal or paternal homolog may be oriented toward either pole of the cell
*A different alignment every time meiosis occurs causes the mixing of alleles in gametes

Question 8

4. Anaphase I

Answer 8

*Homologous chromosomes pairs are pulled apart to the opposite sides of the cell
*Sister chromatids remain attached at the centromere
*The cell elongates

Question 9

5. Telophase I and Cytokinesis

Answer 9

*The nuclear envelope reforms
*The spindle apparatus dissolves and the chromosomes unwind
*The result of telophase and cytokinesis is two haploid daughter cells

Question 10

6. Prophase II

Answer 10

*Meiosis II is the same as mitosis except that the starting cells are haploid and there has been no chromosome duplication.
*During prophase II, chromosomes condense and the nuclear envelope dissolves.
*The centrosomes move to the poles and form the spindle.

Question 11

7. Metaphase II

Answer 11

*Chromosomes line up at the center of the cell in a single file just like in mitosis.
*No pairs!

Question 12

8. Anaphase II

Answer 12

*During anaphase II, the cell elongates.
*Sister chromatids are pulled apart to opposite ends of the cell.

Question 13

9. Telophase II and Cytokinesis

Answer 13

*During Telophase II, the nuclear envelope reforms, and chromosomes unwind
*The spindle apparatus dissolves
*Telophase II ends with four haploid daughter cells

Question 14

Meiosis and Genetic Diversity

Answer 14

Meiosis brings genetic variation in 3 ways:
1. Independent assortment of homologous chromosome pairs during meiosis
2. Crossing over of homologous chromosome pairs
3. Fertilization (combines two distinct genomes)
*In humans (2^23)^2=70, 368, 744, 000, 000 chromosomally different zygotes are possible
*If crossing over occurs only once (4^23)^2=4, 951, 760, 200, 000, 000, 000, 000, 000, 000 different zygotes are possible

Question 15

How traits are inherited

Answer 15

Alleles and chromosomes
*Allele: alternative versions of the same gene
*Locus: where a gene is found on a chromosome
*Homozygous: an organism that has 2 of the same alleles for a given gene
*Heterozygous: an organism that has 2 different alleles for a given gene

Question 16

Interactions and expression of alleles

Answer 16

*An organism’s phenotype is the expression of its genes or its physical or biochemical appearance.
*An organism’s genotype is its genetic makeup.
*A dominant allele (symbolized by a capital letter) is one that expresses its phenotype in the heterozygote
*A recessive allele (symbolized by a lowercase letter) is one that is only expressed in the phenotype when the organism is homozygous (contains 2 recessive alleles).